Avoid problems of inclusions for right boot.img

Adapted shbootimg.mk to tighten all the code in a single function ~ $(INSTALLED_BOOTIMAGE_TARGET).
In fact, while making an otapackage, it used to include the old boot.img (the result of first 10 lines of old shbootimg.mk) and while flashing the zip, we had a bootloop.
New shbootimg.mk has now all the needed code under $(INSTALLED_BOOTIMAGE_TARGET) and then the right - and working/booting - boot.img is included properly in the zip.